In such systems, terrain to be surveyed is subjected to a primary electromagnetic field, and measurements are made of signals induced in a receiver by secondary magnetic fields generated by current induced in the terrain by the primary magnetic field. The secondary magnetic field, in practical situations, comprises (assuming an alternating primary field) components both in phase with and in quadrature with the primary field. A problem with all such systems is that the primary field is very large compared with the secondary field, which leads to difficulty in isolating the desired secondary signals. One approach to these problems in techniques of the frequency domain type, in which both a transmitter which generates the primary field and the receiver are operated at one or more defined frequencies is to provide a receiver in which the current induced by the primary field is cancelled so as to leave only the desired currents induced by the secondary fields. Such cancellation arrangements, if effective, are usually complex and/or critically dependent on the maintenance of a predetermined geometrical relationship between a transmitter of the primary signal and the receiver. Slight variations in this geometry, which are to some degree inevitable, produce inaccuracies of cancellation which show up as noise in the received signal. These problems are of course most serious in relation to the in-phase component of the secondary signal. All of these problems are aggravated by the fact that for most purposes it is desirable to use frequencies that are as low as possible, which still further reduces the relative magnitude of the secondary signal.
An alternative approach has been to utilize so-called transient techniques, in which the primary field is interrupted at intervals, and a potential induced in the receiver representative of the rate of change of the secondary field is sampled at intervals during these interruptions. Since the primary field is unchanging at a zero level during the measurements, the problems associated with cancellation are avoided, and the equivalent of low frequency observations can be achieved merely by sufficiently delaying sampling of the transient signal, but the available time derivatives of the secondary signal components are of relatively smaller magnitude, and become increasingly smaller the longer sampling is delayed, thus again impairing signal-to-noise ratios. It has also been widely believed that such transient techniques only provide data as to the quadrature components of the secondary signal, upon the reasoning that any primary component would be in phase with the primary signal, and thus since no measurements are made in the presence of the primary signal, no measurements are made of the in-phase component of the secondary signal. Whilst this reasoning is fallacious, it is true that known transient techniques fail to discriminate between the in-phase and quadrature components of the secondary signal.
A characteristic of transient systems is that they are essentially broadband in nature since the response will contain components extending through the frequency spectrum. This of course implies a broadband receiver so that decay of the transient secondary signal can be observed for long enough to enable its information content to be exploited. This characteristic also is associated with noise problems.
In his paper "Resolving Capabilities of the Inductive Methods of Electroprospecting" published in Geophysics, Vol. 43, No. 7 (Dec. 1978), pages 1392-1398, one of the inventors discusses the relative theoretical resolving powers of the frequency domain and transient methods, in terms of their capability of distinguishing wanted signals from geological noise. In that paper, he concluded that for multispectral frequency domain systems excellent rejection of the effects of geological noise could be achieved by measuring the low frequency in-phase components of the received signal but that, since these are of low amplitude, this is difficult to carry out for the reasons given above.